The present invention relates generally to methods applied in web-forming sections of paper machines.
Specifically, the present invention is directed to a web forming and dewatering method applied in web-forming section which comprises a lower-wire loop situated in proximity to a headbox of the paper machine so as to form an initial, preferably substantially horizontal, single wire run and zone in which the dilute fiber suspension constituting the web-forming stock is dewatered through said lower wire by dewatering means, the web-forming section also comprising an upper wire loop guided by guide and/or web-forming rolls and having a joint run with a subsequent run of the lower wire so as to form a two-wire dewatering zone within which dewatering of the web takes place substantially upwardly through the upper wire.
Fillers, normally constituted by mineral substances, are often incorporated within paper mainly in order to improve the and printability properties of the paper. As is well known, the addition of fillers can be accomplished in two ways, i.e., either as a filling into the pulp stock or by means of coating. In the former procedure, the filler is added into the pulp as a sludge before the pulp arrives at the paper machine so that the filler is present in the ready paper mixed in the entire fiber material. In the latter procedure, an appropriate sizing agent, i.e., starch or caseine, is mixed with the filler in a water phase, whereupon the surface of the paper is coated with the mixture.
The fillers are usually added into the fiber stock in the form of a water sludge. The addition of the fillers takes place, e.g., into the pulper, grinders, or proximate to the headbox of the machine, into an appropriate pulp chest or onto the inlet side of the headbox feeding pump. Fillers are used most commonly for printing papers, for improving their opacity, whiteness, ink-absorption, and smoothness. Moreover, fillers have a particularly favorable effect on the quality of paper to be glazed.
However, fillers do not adhere well to the fiber themselves of the stock which is a main reason for the poor retention of fillers in the ready paper. For this reason the filtering effect of the fiber network which withholds the filler particles becomes an important factor affecting the retention of the fillers. The degree of filtering effect provided by the fiber network is determined by the thickness of the fiber web running on the wire, by the density of the fiber network, by the mesh of the wire and, moreover, by the draining or dewatering effects applied to the web. The grinding, which fibrillates the fibers, improves the retention of fillers by promoting the formation of the fiber network and thus the adhesion of the fillers to the fibers.
The retention is also affected by the physical properties of the filler particles, such as their size, shape and density. Larger particles are filtered better than smaller ones which are readily carried through the filtering fiber layer. Heavier particles are filtered to a lesser extent than lighter ones.
Like all the fine materials in paper, such as fine fibers and coloring agents, fillers tend to be unevenly distributed across the thickness of the paper thereby causing a so-called two-sidedness in the paper which means that the two sides of the paper have different surface properties. The two-sidedness of paper manufactured in the fourdrinier machines results from the fact that the fillers are washed away from the lower portion of the fiber web along with the drained water whereby the top portion of the web becomes enriched in fillers relative to the bottom portion of the web.
As is well known, attempts have been made to relieve the problems of two-sidedness of paper, not only by means of additives improving the retention of the fillers in the fiber network, but also by performing the dewatering during the initial filtering stage gently which requires a prolonged draining time and, consequently, either a lengthening of the wire section or reduction in the speed of the paper machine.
In the case of fourdrinier machines, the difficulties of distribution of fines and fillers are encountered in the manufacture of papers for offset printing. A high content of fillers and fines in the upper face of the paper causes dusting, which is a serious drawback in the offset process. On the other hand, papers manufactured by means of twin or two wire machines are considered well suited for offset printing due to the symmetric distribution of fines within the paper which is a result of the substantially equal washing of both faces of the web during the two-sided dewatering. Thus, it is recognized that due to the more uniform distribution of fines, offset printing on paper manufactured by means of a twin-wire method is more successful than printing on paper manufactured by means of a fourdrinier machine. The quality of offset printing is becoming increasingly important since the letter-press printing method is being increasingly replaced by offset printing.
On the other hand in a twin-wire former, the filler content of the faces of the paper web cannot in all cases be brought to the desired level and when fourdrinier wire sections are used, only the upper side of the paper web, i.e., the side facing away from the wire, will have a satisfactory filler content. An unduly low filler content of the web faces is particularly problematical in the case of super calendered gravure printing papers. Although attempts have been made to increase the filler content of the paper faces by increasing the filler content of the stock in the headbox, such attempts have not proved entirely satisfactory due to the typically poor retention of the fillers as discussed above and the enriched amount of fillers in the interior of the paper. Moreover, consistency of the stock in the headbox becomes easily excessive which deteriorates the formation of paper.
In conventional twin-wire formers, or so-called full-gap formers, which are now in common use, the stock is supplied into the wire section as a thin suspension whereupon a violet dewatering of the stock in both direction begins immediately or after a very short single-wire section, the result of this is that a considerable quantity of filler agents which have been added to the pulp, as well as fine fibers are washied away from the web along with the water being drained therefrom. Of course, this results in a considerable deterioration in the quality of the paper and, in particular, impairs the very properties intended to be provided to the paper by means of the fillers. Moreover, a dewatering takes place simultaneously in two directions also results in a weakening of the mid-portion of the paper web which in turn results in a low internal bond strength.
In a two-wire former which is disclosed in Finnish Pat. No. 50,648, assigned to applicants' assignee, a dewatering process is applied to avoid the drawbacks discussed above. This two-wire former is characterized by a single-wire initial portion of the wire part which is sufficiently long so that while a gentle dewatering takes place in the initial portion, the fiber web has time to obtain such a degree of felting prior to a two-wire portion of the wire part that the fibers can no longer be significantly shifted with respect to each other. Moreover, the two-wire portion of the wire part is guided, by means of a draining roll or by a draining box, so as to be curved downwardly whereby water is drained by the effects of centrifugal force and of the pressure zone produced by the tensioning between wires in the curved portion through the upper wire in a direction opposite to the direction of dewatering taking place in the single-wire initial portion. The main objective is to reduce the removal of additives, such as fillers, and fines from the fiber web and to increase the internal bond strength of the paper being manufactured.
It is well-known that in a conventional fourdrinier wire section, dewatering of the web is arranged to take place only in the downward direction so that fines and filler agents escape due to the washing effect of the foils or table rolls from that side of the web which faces the wire. For this reason a paper web manufactured in such a fourdrinier machine is anisotropic in regard to the surface properties of its two sides, the upper side of the web being smoother and containing more fines and fillers than the lower or wire side. Moreover, the wire side of the web is left with a mark from the wire.
For the above reasons, paper made by means of two-wire formers is considered superior, especially with respect to printing properties compared to paper made on fourdrinier wire sections. In such prior art two-wire formers in which no stationary dewatering elements are utilized, formation is usually poor and no pulsations of the dewatering pressure can be produced which would improve the formation. Another drawback of such prior art formers is that the same are provided with means for adjusting the ratio of the quantities of water being dewatered through the upper and the lower wire. The desirability of providing the capability for such an adjustment has been expressed on several occasions.
Two-wire formers are also known in the art wherein the dewatering is mainly effected by stationary dewatering elements. However, in such prior art two-wire formers a drawback is present in that filler and fine retention is relatively poor whereas wire wear and power consumption is high.
Recently, modernizations of fourdrinier wire sections have become common in which one or more upper-wire units are assembled above the fourdrinier wire section so as to render possible an upward dewatering from the web with the objective of both increasing the dewatering capacity as well as improving web formation and retention of fillers and fines. An increased dewatering capacity in turn permits an increase in the speed of the paper machine. A further aim of such modernized fourdrinier wire sections is to provide the capability of reducing the consistency of the stock supplied from the headbox which itself is advantageous. In certain cases, old low-speed newsprint machines have beer converted or modernized into board machines which produce thick paper and board grades requiring a high dewatering capacity without increasing the speed of the machine.
As examples of prior art arrangements of the type described above, reference is made to Finnish Patent Application No. 78 2709 (Beloit Walmsley Ltd.) and to British Pat. No. 1,582,342 (Australian Manufacturers Ltd. and Beloit Walmsley Ltd.). Reference is also made to U.S. Pat. No. 4,154,645 and to Finnish Patent Application Nos. 81 0373 and 81 1514, all assigned to applicants' assignee.
With respect to the prior art technology related to the present invention, reference is further made to published Swedish Patent Application No. 308,244 and Finnish Pat. No. 40,436.
A web-forming section including an initial single-wire dewatering zone followed by a two-wire dewatering zone may be designated a "hybrid" former. Hybrid formers are disclosed in U.S. Pat. Nos. 3,846,233; 4,154,645; 4,220,502; and 3,994,774.